1. Field of the Invention
The present invention relates to data transmission over optical waveguides, and more particularly to a method for inserting an additional optical channel into a main optical channel transmitting data between two stations.
2. Description of the Prior Art
It has become commonplace to transmit data between two stations using optical waveguides, which allow for the transmission of data over very long distances without the need for intermediate amplifiers. Optical waveguides have very wide bandwidths and very low attenuation of the transmitted light signals. Another advantage of optical waveguides is that the data transmission is not affected by external electrical or magnetic noise fields. The present technology allows data to be transmitted at very high transmission or bit rates as, for example, nominal rates of 34 M bit/second and 140 M bit/second.
Data is normally transmitted using standardized codes having fixed rules. These codes are used to prevent d.c. buildup on electrical transmission lines and to eliminate excessively long strings of zeroes which can hamper clock recovery. The data is usually provided in electrical form conforming to a known electrical coding system and is thereafter recoded in a transcoder to a corresponding optical code for transmission over the optical waveguides. At a receiving station, the optical code is recoded into the electrical code in another transcoder.
A typical electrical code may be the ternary AMI code having three different states, which electrical code would be recoded into an optical code such as the CMI code for optical transmission. The CMI code is a binary non-return to zero code. Another electrical code in common use is the HDB3 code which is recoded into the MCMI optical code.
With these known optical transmission procedures, a significantly larger number of channels can be transmitted with the use of significantly fewer optical waveguides, as compared to the conventional technology used with metallic conductors. Nevertheless, additional capacity may be required, and additional channels may be needed on existing transmission paths without affecting the transmitted data.